Roof cutting is an effective technique for controlling the deformation and failure of the surrounding rock in deep gob-side entry. The determination of the roof cutting parameters has become a popular research subject. Initially, two mechanical models are established for the non-roof-cutting and roof-cutting of gob-side entry in deep mining conditions. On this basis, the necessity and significance of roof cutting is revealed by analysing the stress and displacement of roadside prop. The Universal Distinct Element Code numerical simulation model is established to determine the key roof-cutting parameters (cutting angle and cutting height) according to the on-site situation of No. 2415 headentry of the Suncun coal mine, China. The numerical simulation results show that with the cutting angle and height increase, the vertical stress and horizontal displacement of the coal wall first increase and then decrease, as in the case of the vertical stress and displacement of roadside prop. Therefore, the optimum roof cutting parameters are determined as a cutting angle of 70° and cutting height of 8 m. Finally, a field application was performed at the No. 2415 headentry of the Suncun coal mine. In situ investigations show that after 10 m lagged the working face, the stress and displacement of roadside prop are obviously reduced with the hanging roof smoothly cut down, and they are stable at 19 MPa and 145 mm at 32 m behind the working face, respectively. This indicates that the stability of the surrounding rock was effectively controlled. This research demonstrates that the key parameters determined through a numerical simulation satisfactorily meet the production requirements and provide a reference for ensuring safe production in deep mining conditions.

Phosphoric triamide (PTA) and glycidyl polyhedral oligomeric silsesquioxane (POSS) were simultaneously incorporated into the cured network of a bisphenol F epoxy resin and 4,4′-diaminodiphenyl methane (DDM) to improve the thermal stability and flame retardancy. PTA was synthesized by triethyl phosphate and DDM, and its chemical structure was confirmed by ¹H nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR). The differential scanning calorimetric (DSC) results showed that the introduction of PTA and POSS slightly increased the glass transition temperature of the epoxy resin. The thermogravimetric analysis results indicated that compared with the pure, phosphoric, and silicic epoxy resins, the modified epoxy resin possessed the lowest weight loss rate and highest char residue. Its limiting oxygen index value was as high as 30.5, and the UL-94 grade reached V-1. A decomposition test was carried out to obtain sufficient char residue and investigate the condensed mechanism. The scanning electron microscopic images demonstrated that the char residue of the modified epoxy resin had a compact structure. The energy dispersive X-ray and FTIR analyses verified the synergistic effect of the phosphorus and silicon in the PTA and POSS, respectively, on the epoxy resin.

Carbon fibers were surface treated with supercritical water/nitrate system to improve the interfacial adhesion of the carbon fiber/epoxy composite. The surface chemistry analysis showed that oxygen functional groups on the surface of the carbon fibers increased after treatment, which were mainly carbonyl and carboxyl groups. The surface microstructure observation indicated that the treatment obviously increased the surface roughness of the carbon fibers. Surface energy of the treated carbon fibers also increased. The increased functional groups, surface roughness and surface energy were beneficial to enhance the interfacial adhesion of the carbon fiber/epoxy composite. Compared with the untreated carbon fibers, the strength loss of the treated carbon fibers was less than 3% and the other mechanical properties were almost unchanged. The caclulated interfacial fracture energy and the interfacial shear strength of the treated carbon fiber/epoxy composite were enhanced by 19% and 29%, respectively, compared with the untreated carbon fiber/epoxy composite. In summary, supercritical water/nitrate treatment is a potential method for changing the inert surface of carbon fibers to improve the interfacial adhesion between carbon fibers and matrix.

The use of carbon fiber-reinforced resin matrix composites instead of metals to manufacture cryogenic propellant tanks for spacecrafts is a development trend in the world aerospace industry. Cryogenic mechanical properties of the composites should be investigated in detail due to that the ultra-low temperature environment may cause micro cracks in the composites, leading to propellant leakage. In the present study, cryogenic tensile properties of a carbon fiber reinforced silicon-containing epoxy resin aomposite are investigated using experimental and numerical simulation methods. A silicon-containing epoxy resin with excellent cryogenic mechanical properties is developed by introducing a synthesized organic silicon polymer epoxidized polysiloxane and Nano-SiO₂ into bisphenol F type epoxy resin. The tensile strength and modulus of the silicon-containing epoxy resin at −180 °C are 202.63 MPa and 7.81 GPa, which are 16.71% and 3.03% higher than those of bisphenol F type epoxy resin. The tensile strength of the silicon-containing epoxy resin at −180 °C is increased by 107.7% compared to that at room temperature, and the modulus at −180 °C is nearly twice that at room temperature. The carbon fiber reinforced silicon-containing epoxy resin composite is prepared by vacuum injection molding. The finite element model for the representative volume element of the composite unidirectional plate is established. The random sequence expansion method is used to randomly distribute the carbon fibers and simulate the thermal residual stress, the elastic performance, and the damage of the composite at cryogenic environments. Through comparison, it is found that the simulation results are in good agreement with the experimental results. The simulation reliability for cryogenic mechanical properties of carbon fiber reinforced resin matrix composites is verified. It is expected to provide a reference for the analysis and evaluation of cryogenic mechanical properties of composite tanks.